So you can get the M Performance Power Kit for a 320d but we cant get it for the 328 in the US lol

The US and europe is like black and white. Two tottaly different case..
In EU fuel consumption, traffic and legal things are much more strick!
This has nothing to with the MPK but there must be a reason! Why you cant get it!

Maybe the 420d has more power limited than the 328i!
MPK gives around 16-20hp for diesel and in my opinion it would give 5-10?!

Dont want to be smart ass or anything but maybe this could maybe be 1 reason!